The production of fumed silica from chlorosilane feedstocks is well known in the art. The combustion of tetrachlorosilane with hydrogen and air in a pyrogenic burner is known. See for example Degussa, U.S. Pat. No. 4,276,274, German Patent 974,793, U.S. Pat. No. 2,990,249, U.S. Pat. No. 3,086,851, U.S. Pat. No. 3,006,738, German Published Application 2,153,671 corresponding to U.S. Pat. No. 3,954,945.
Where residual gasses are cooled with quench air, elevated oxygen levels are present. Typically, oxygen enriched residual gas streams produced by prior art methodologies include a range of components such as an aerosol oxide, about 0.5% free chlorine, less than about 15% oxygen, minor quantities of hydrogen, nitrogen, carbon dioxide, and trace materials in balance. After the fumed silica is removed, and the residual gas stream is introduced into an aqueous HCl absorber, the stream is either introduced into a caustic scrubber for chlorine removal, into an incinerator, or is vented into the atmosphere. Caustic scrubbing and incineration methodologies are expensive and include lengthy maintenance periods due to the corrosive nature of the gasses. Venting to the atmosphere is objectionable due to the economic loss associated with the foregone sale of hydrochloric acid.
Several attempts have been made to reduce chlorine emissions. For example, the use of low molecular weight hydrocarbons and hydrogen as reactants for the reduction of chlorine is disclosed in U.S. Pat. No. 3,485,577. Ore oxide is combined with a carbonaceous reducing material and reacted with chlorine gas. Oxygen released from the ore oxides is depleted by the carbonaceous material, forming residual gasses comprising unreacted chlorine, carbon dioxide, and carbon monoxide. Hydrogen or gaseous hydrocarbon is introduced by continuous or intermittent mixing systems including pressure reducing valves and pressure gauges. This approach is sensitive to residual gas temperature fluctuations; operating in a recommended range of 400.degree. to 1300.degree. C. and a preferred range of 800.degree.-1300.degree. C. The preferred temperature range is relatively high due to the depleted oxygen level in the system. In the recommended temperature range, chlorine would compete with oxygen for available reducing agents. In elevated oxygen systems, the available hydrogen would react preferentially to form water vapor in parts of the recommended and all of the preferred temperature ranges leaving chlorine in the residual gasses. More precise temperature control would be required to preferentially remove chlorine rather than oxygen. Where gaseous hydrocarbons are dispersed in the residual stream, the process of the '577 patent results in the presence of undesirable chlorinated hydrocarbons. A process for removing chlorine in a broad range of residual gas temperatures is therefore desirable.
The '274 Degussa patent pertains to a process for reducing the chlorine emissions in the residual gas stream generated by the hydrolytic combustion of volatile metal halides. Temperature is controlled to a point below which the reduction of chlorine by hydrogen is favored over the formation of water vapor. An excess stoichiometric amount of hydrogen of about 2.5:1 moles of chlorine is disclosed. The hydrogen is added at residual stream temperatures of from about 932.degree. to 1292.degree. F. by single or multiple port dispersion devices such as molded nozzles. Although approximately 93% chlorine conversion is achieved, the process is very sensitive to temperature fluctuations.
A process for nearly quantitative chlorine conversion in processes employing an air quench would be a desirable advancement in the art.
A further process for dehalogenating residual gasses is disclosed in U.S. Pat. No. 4,347,229 where a mixture of hydrogen and nitrogen is dispersed by means of a double jacketed pipe at temperatures between about 1022.degree. to 1166.degree. F. The pipe has two series of bores in the jacket through which the hydrogen can be introduced into the residual gas stream.
German Patent 1,244,125 pertains to a process to control chlorine in the residual gas stream where a 50 to 80% proportion of the residual gasses are recycled into a closed burner chamber thereby eliminating the addition of secondary air to the reactants. Processes where the burner conditions are modified often result in product variability and are therefore undesirable.
A process for treatment of residual gasses which results in reduced halogen and halogenated hydrocarbon emission levels in oxygen containing streams would be a significant improvement in the art.
Another object of the present invention is a process for producing high quality, finely divided oxides of metals or metal halides with residual gasses which ultimately contain reduced halogen levels, which uses conventional process equipment, and which operates in a broad temperature range.
Another objective of the present invention is a process for producing high quality, finely divided oxides of metal or halides with reduced halogen gas emissions by rapid dispersion of gaseous hydrocarbon and rapid dispersion of hydrogen by means which favor the reduction of free halogen and halogenated hydrocarbons.
Other objects will become known hereafter to those of ordinary skill in the art through the following specification, drawings, and claims as hereinafter provided.